It is well known to the art that condensation of a phenol and an aldehyde provides materials curable to thermoset phenolic resins. Base-catalyzed condensation employing at least about a stoichiometric amount of aldehyde provides condensates known as resoles whereas acid catalysts and a deficiency of aldehyde provides novolacs. Characteristic of both liquid and solid resoles is their heat-curability to fully cross-linked, infusible products without the need for an added cross-linking agent. From this standpoint, resoles are more descriptively referred to as One-Step phenolic resins in contrast to novolacs or Two-Step resins which do require the addition of a cross-linking agent for the curing process. The reactivity of resoles and ability to self-condense to higher molecular weight resins is attributable to the presence of hydroxymethyl groups which become bonded to the aromatic phenolic nucleus during the base-catalyzed condensation.
Curing of resoles to higher molecular weight, cross-linked thermoset resins proceeds with generation of heat and is accelerated by acid materials. In the presence of strongly acidic accelerators of the exothermic reaction and a source of blowing action, liquid resoles cure rapidly to cellular phenolic resins. Although phenolic foams are used to embed floral arrangements and for general packaging purposes, they have not found the widespread industrial application enjoyed by cellular polyurethanes. As compared with cellular polyurethanes, phenolic foams possess an outstanding inherent resistance to burn with an open flame, and emit very low levels of smoke on heating. Consequently, greater attention is being focused on phenolic foams for a variety of end-use applications where fire is especially hazardous. In addition to improving certain properties of phenolic foams such as their friability and punking behavior (that is, glowing combustion without a visible flame), there is also a need to overcome certain drawbacks associated with the resole raw materials such as, in particular, their poor shelf-life. From the standpoint of commercial application, the most significant resoles are those derived from phenol itself and formaldehyde.
As generally described in the literature, formation of phenol-formaldehyde resoles comprises condensation of phenol with at least about an equimolar amount of formaldehyde in the presence of an alkaline metal catalyst and treatment of the condensate with an acid such as sulfuric acid to neutralize the alkaline catalyst. Depending upon the end-use application of the resole product, the water content of the neutralized condensate may be adjusted to a predetermined level either by dilution or dehydration. If an alkali metal hydroxide is employed as the catalyst, the condensate may require filtration to remove precipitated salts formed during neutralization depending upon the water content and end-use application of the resole product. Irrespective of whether the neutralized condensate is filtered, alkali metal salts are present in the product. In any event, in the commercial manufacture of phenol-formaldehyde resoles supplied as foamable resins, filtration is avoided by the employment of barium hydroxide catalyst which, when neutralized with sulfuric acid, forms a salt which does not precipitate and is left in the product.
The exact composition of phenol-formaldehyde resoles has not been elucidated. However, it is generally recognized that they contain varying amounts of mono-, di- and tri- methylolated mononuclear phenols (phenol alcohols) as well as polymethylolated polynuclear phenols. The methylol groups are activated by the presence of phenolic hydroxyl groups and consequently conventional resoles undergo facile self-condensation, even at room temperature, which contributes to their poor shelf-life.
The problem of resole instability is a longstanding one and is associated with commercially available phenol-formaldehyde resoles. It is of particular concern with respect to liquid resoles of low water content supplied as foamable resins. Thus, unless many grades of liquid resoles are employed by the foam manufacturer within a relatively short period of time after their preparation, they gradually lose their initial reactive thermosetting character. For example, resoles designed for manufacture of rigid phenolic foam of medium to high density grade (about 1.5 to about 30 pounds per cubic foot) by controllable pour-in-place reactions (free-rise or molded), "deaden" as foamable compositions after about four or less weeks storage at ordinary ambient temperatures (e.g., about 20.degree.-25.degree. C.). It is evident that gradual loss in reactivity during storage hampers standardization of foam formulations to achieve foamed products of predetermined specifications such as foam density. In order to slow their tendency to advance to a higher molecular weight and less reactive form, such resoles specified for manufacture of rigid phenolic foam of medium to high density grade, are maintained at below ordinary ambient temperatures such as 5.degree. C. and are often refrigerated during storage and shipment. Installation and maintenance of refrigeration or cooling equipment at the foam manufacturing plant site for storage purposes is, of course, costly and troublesome.
Other commercially available resoles are designed for manufacture of very low density foam (0.2 to about 0.4 pound per cubic foot) and have a substantially higher content of free phenol which, generally, is from about 20 to about 40 weight percent. Such specialty resoles are reported as storable for reasonable lengths of time (maximum of three months at 24.degree. C.), although for maintenance of foam performance, the resole manufacturer recommends that they be kept at temperatures below 21.degree. C. and that extended storage be avoided. In view of their highly reactive nature in the presence of acidic catalysts of conventional foam formulations, foaming at less than ambient temperature is required in order to control the curing process and obtain a product of satisfactory quality. Ordinarily, therefore, such resoles are blended with other less reactive materials or as such are suitable for spray application of phenolic foam.
It is desirable and is a primary object of this invention to provide base-catalyzed condensates of a phenolic compound and an aldehyde, particularly phenol-formaldehyde resoles, having improved aging characteristics.
Another object is to provide resoles which, in addition to extended shelf-life, are especially useful in providing cellular phenolic products.
Another object is to provide phenol-formaldehyde resoles which substantially retain their reactivity as foamable compositions during prolonged storage at ambient conditions and which are capable of being foamed to cellular phenolic products of a wide density range including densities as low as about 0.5 pounds per cubic foot, by controllable pour-in-place foaming reactions.
A further object is to provide a method for producing resoles having the aforesaid characteristics.
Various other objects and advantages of this invention will become apparent to those of ordinary skill in the art from the accompanying description and disclosure.